As well as almost certainly being a condition of any official permission and insurance, a proper consideration of the risks involved in putting on your event is a sensible part of the process of planning your event. You have a duty of care to your spectators and competitors to ensure that reasonable precautions are taken to avoid unneccessary risks, and they have a right to expect that reasonable precations have been taken.

Rule and regs

Decide on the rules. How much detail you need to go into depends on whay type of event you are planning to run. A kids push race around the local playing fields will not need as detailed a set of rules as a full on gravity race. Any rules you set should be ones you are prepared to enforce.